1. General Purpose of the Invention
The present invention relates to a power supply circuit; and more particularly to a power supply circuit for a monitor having a switching mode power supply (SMPS) circuit with a Power Factor Correction circuit (PFC).
2. Description of Related Art
Traditionally, a PFC circuit is used in devices with high wattage, which improves the power of an alternating-current transmission system so the voltage and the current are substantially in phase. But an increasing number of lower wattage devices, like personal computers (PCs) having display monitors, provide a remarkable potential for saving power by equipping them with a PFC circuit. Display monitors may also include other display devices which can be connected to a mains voltage and based, for instance, on cathode ray tube, liquid crystal or projection-type display technology.
It is known in the art to use a separate PFC circuit between a mains outlet and a power consuming device. For example, EP 0 673 106 A1 discloses circuits for achieving a higher power factor in a switched mode power supply. FIG. 2 of EP 0 673 106 A1 shows a circuit having a step-up regulator arranged between a bridge-type rectifier and an AC energy distribution source that includes a capacitor C, a transformer M, a controller CTRL1, a switch S1, a diode D2 and a capacitor C2. The reader is also referred to a technical disclosure by Siemens identified as the TDA 4862 datasheet.
It is also known in the art to add a PFC module inside a housing of such a power consuming device.
Moreover, many devices have a universal power supply, i.e., the same power supply is able to operate with a large value of the mains voltages, for instance, in U.S. (110V) and in Europe (230V).
There is a need in the art to: (1) find an economical solution where additional control circuitry for the PFC circuit is minimized by sharing an existent power supply controller for controlling power factor correction; (2) make a PFC circuit controllable according to the mains voltage of the display monitor; and (3) equip display monitor power supplies with a PFC circuit which are also universal, i.e., properties of a PFC circuit are controlled according to the operating situation. The PFC inductor is large, heavy and expensive and there is a need to do PFC without adding an extra inductor.
In the art, universal monitors which cover both mains voltage types are working typically with a mains voltage range of 90V, . . . , 265V.
FIG. 1 (Prior Art): Basic Power Supply Circuit for a Display Monitor
In particular, FIG. 1, including FIGS. 1A and 1B, shows a basic power supply circuit generally indicated as 100 for a display user interface such as a display monitor 95, which includes a mains filter 1, a mains rectifier 10, a power factor correction circuit 30, a main power supply 40, deflection circuits 60, 70, a video amplifier 80 and a microcontroller 90.
In FIG. 1A, the mains rectifier 10 provides a mains rectifier signal having a current and voltage having a certain phase relationship.
The power factor correction circuit 30 includes a PFC controller 31, an inductor 32, a diode 33, a capacitor 34, a control line 35 and a switch 36. The PFC circuit 30 and also the PFC controller 31 are known in prior art. See the supply voltage generation in Siemens' FIG. 3 of the TDA 4862 datasheet, which is a known prior art method. The PFC controller 31 monitors both the incoming full-wave rectified mains voltage and the voltage in the capacitor 34. The product of these voltages is used to control the pulse ratio of the switch 36 so that the waveform of the current drawn from the mains corresponds to the waveform of the voltage. In FIG. 1, the combination of a transformer 37, a diode 38 and a capacitor 39 provides for the voltage generation to the PFC circuit 30. In operation, the alternating flux of the coil 32 induces alternating voltage in the winding 37, which is rectified by the diode 38. One disadvantage of the display monitor power supply circuit 100 is that it needs extra components included in the PFC circuit 30.
In FIG. 1B, the main power supply 40 includes a switching mode power supply (SMPS) controller 41, a transformer 42, a switch 43, a feedback loop 44, an isolator 45, diodes 46a, 46b, 46c, a feedback resistor 47 and a reference voltage diode 48. The feedback loop 44 is connected to a standby state control line 44a from the microcontroller 90. All of these circuit components are all known in the art, the scope of the invention is not intended to be limited to any particular type thereof, and a person skilled in the art would appreciate how they cooperate to provide a main power supply. The function of the main power supply 40 will be explained in more detail below and further in relation to the description of FIGS. 2-6 below.
In operation, the SMPS controller 41 controls energy supplied to primary winding 42a of the transformer 42, by switch 43, which is preferably a Field Effect Transistor (FET). The SMPS controller 41, can control, for instance, switching frequency and/or duty cycle. Feedback along line 44, from the secondary side of the transformer 42, is fed to the SMPS controller 41, in order to keep rectified secondary voltages V1, V2, V3 stable. The display monitor circuits 60, . . . , 95 are all fed by the power supply.
Regarding the isolated/non-isolated interface, all parts in a given high wattage device which have calvanic connection to parts which the user can touch must be isolated from the mains voltage. Such isolation is realized with the transformer 42, having adequate isolation between primary and secondary windings. The feedback signal 44 is fed from the secondary to primary winding via isolator 45, which may be an opto-isolator.
There are disadvantages to the basic display monitor power supply circuit 100 shown in FIG. 1, which result primarily from the separate PFC circuitry 30. The inductor 32 is large, heavy and expensive. The switch 36 must be a powerful switch, and the controller circuit 31 is also needed.
The present invention provides a solution to the aforementioned problems in the industry and also fulfills the aforementioned needs in the industry.